Method and apparatus for performing a fine timing measurement in a wireless network

ABSTRACT

A first wireless communication device includes a timing module to provide a timing synchronization function (TSF) time. A fine timing measurement module is to transmit a first request to perform a fine timing measurement to determine a distance between the first wireless communication device and a second wireless communication device. The first request includes a first value of the TSF time at which to start a burst period of the fine timing measurement, and the first value is determined in accordance with the TSF time provided by the timing module. The fine timing measurement module is further to receive a first response as transmitted from the second wireless communication device in response to the first request and transmit a second request to start the burst period at a TSF time indicated by the first value. The first response includes the first value of the TSF time.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present disclosure is a continuation of U.S. patent application Ser.No. 14/690,988, filed on Apr. 20, 2015, which claims the benefit of U.S.Provisional Application No. 61/948,364, filed on Mar. 5, 2014. Theentire disclosures of the applications referenced above are incorporatedherein by reference.

FIELD

The present disclosure relates to systems and methods for performing arange measurement in a wireless network using a fine timing measurementprocedure.

BACKGROUND

The Institute of Electrical and Electronics Engineers (IEEE) hasdeveloped several 802.11X specifications that define communicationprotocols used by network devices operating in wireless local areanetworks (WLANs). For example, the communication protocols includeauthentication schemes that can be used to securely exchange databetween the network devices. The communication protocols includepower-saving strategies that can be used to save power in the networkdevices. The communication protocols include synchronization schemes tosynchronize clocks of the network devices, and so on. Some protocolsinclude a fine timing measurement (FTM) procedure used to measure thetime of flight (ToF) of a radio frequency (RF) signal between twonetwork devices. The ToF is used to measure a range (i.e., distance)between the network devices.

FIG. 1 shows an example FTM procedure used to measure ToF between twonetwork devices—client stations STA1 and STA2—operating in a WLAN. STA2initiates an FTM request by sending a request frame (FTM request) toSTA1. STA1 receives the FTM request at time t0 and responds by sending acorresponding acknowledgment frame (ACK). The FTM request and thecorresponding acknowledgement frame are followed by STA1 sending an FTMresponse frame (FTM_1) to STA2. The FTM response frame leaves a transmitantenna of STA1 at time t1 and arrives at a receive antenna of STA2 attime t2. STA1 and STA2 may measure timestamps indicative of when the FTMresponse frame leaves the transmit antenna from STA1 (t1) and when theFTM response frame arrives at the receive antenna at STA2 (t2). STA2responds by sending an acknowledgement frame to STA1. The ACK frameleaves a transmit antenna of STA2 at time t3 and arrives at a receiveantenna of STA1 at time t4. STA2 and STA1 measure timestamps indicativeof when the ACK frame leaves the transmit antenna of STA2 (t3) and whenthe ACK frame arrives at the receive antenna of STA1 (t4).

The FTM response frame includes several parameters that describe therange measurement bursts, including a burst offset field (i.e., a fieldthat indicates a value of a burst offset). STA2 transmits the FTMrequest to trigger the first FTM burst at a time indicated by the burstoffset field of the FTM Response. The burst offset corresponds to aduration (e.g., 10 ms) between reception of the FTM request by STA1 anda start of the first burst period. Accordingly, the burst offsetprovides an indication, to the stations STA1 and STA2, of the start ofthe first burst period. The stations perform the FTM based on framestransmitted during the burst period. The burst period corresponds to aduration between a start of one burst period (e.g., burst period 1) to astart of a next burst period (e.g., burst period 2).

For example, STA2 transmits another FTM request to STA1 at the start ofthe burst period 1 to notify STA1 that STA2 is ready to receive FTMframes for range measurement. STA1 transmits an ACK and then provides t1and t4 to STA2 in an FTM (FTM_2). STA2 calculates a round trip time(RTT), which is twice the ToF between STA1 and STA2, asRTT=(t4−t1)−(t3−t2)=(t2−t1)+(t4−t3). The ToF between STA1 and STA2 isRTT/2. STA2 can perform additional calculations based on transmittedframes and respective times in burst period 2.

SUMMARY

A first wireless communication device including a timing module and afine timing measurement module. The timing module is configured toprovide a first timer value. The fine timing measurement module isconfigured to receive, from a second wireless communication device, afirst request to perform a fine timing measurement to determine adistance between the first wireless communication device and the secondwireless communication device, and transmit a first response including afirst time to start a burst period for performing the fine timingmeasurement. The first time is one of included in the request receivedfrom the second wireless communication device and based on the firsttimer value. The fine timing measurement module is further configured toperform the fine timing measurement in the burst period starting at thefirst time.

A method of operating a first wireless communication device includesproviding a first timer value, receiving, from a second wirelesscommunication device, a first request to perform a fine timingmeasurement to determine a distance between the first wirelesscommunication device and the second wireless communication device,transmitting a first response including a first time to start a burstperiod for performing the fine timing measurement, wherein the firsttime is one of (i) included in the request received from the secondwireless communication device and (ii) based on the first timer value,and performing the fine timing measurement in the burst period startingat the first time.

Further areas of applicability of the present disclosure will becomeapparent from the detailed description, the claims and the drawings. Thedetailed description and specific examples are intended for purposes ofillustration only and are not intended to limit the scope of thedisclosure.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an example a fine timing measurement (FTM) procedure.

FIG. 2 illustrates an example mismatch in an FTM procedure.

FIG. 3 illustrates an example mismatch between respective burst offsetscaused by a mismatch between FTM requests and FTM responses.

FIG. 4 is an example wireless local area network including one or morewireless communication devices.

FIG. 5 is an example wireless communication device.

FIG. 6 illustrates an example FTM procedure using an absolute burststart time.

FIG. 7 illustrates another example FTM procedure using an absolute burststart time.

FIG. 8 illustrates steps of an example method for performing an FTMprocedure.

In the drawings, reference numbers may be reused to identify similarand/or identical elements.

DESCRIPTION

FIG. 2 shows an example mismatch in an FTM procedure between stationsSTA1 and STA2. In FIG. 2, respective station management entity (SME)sublayers and medium access control (MAC) sublayers of the stations STA1and STA2 are shown to illustrate additional communication between thesublayers. Transmission delays (e.g., delays due to a busy transmissionmedium) may cause an FTM request timeout prior to STA2 receiving the FTMResponse frame from STA1. The FTM request timeout may cause a mismatchbetween FTM requests transmitted by STA2 and corresponding FTM responsestransmitted by STA1. For example, the FTM request timeout may cause STA1and STA2 to operate according to different burst offsets.

For example, STA2 transmits a first FTM request (FTM Request 1) to STA1,which is relayed from the SME to the MAC of STA2 and then provided tothe SME of STA1 via the MAC of STA1. The SME of STA1 responds with afirst FTM response (FTM Response 1) provided to the MAC of STA1.However, the MAC may not be able to access the transmission medium(e.g., the transmission medium may be busy). Accordingly, the MAC ofSTA1 waits for the transmission medium to become free.

At STA2, the first FTM request may timeout if the FTM response is notreceived within a predetermined period. In the event of an FTM requesttimeout, STA2 may transmit a second FTM request (FTM request 2). Asshown, STA1 receives the second FTM request prior to transmitting thefirst FTM response. If the transmission medium becomes free, STA1 maysubsequently transmit both the first FTM response and the second FTMresponse to STA2. Accordingly, a mismatch occurs between the FTMrequests transmitted by STA2 and the FTM responses transmitted by STA1.

FIG. 3 shows an example mismatch between respective burst offsets ofSTA1 and STA2 caused by a mismatch between FTM requests and FTMresponses. For example, STA1 may determine a first burst offset (BurstOffset 1) according to the first FTM request. However, if the MAC ofSTA1 does not transmit the first FTM response (e.g., due to thetransmission medium being busy) prior to the FTM request timeout atSTA2, STA2 may transmit the second FTM request. STA1 may discard thesecond FTM request, and subsequently transmit the first FTM responseupon the transmission medium becoming free. Accordingly, STA1 operatesaccording to a first burst period (Burst 1) calculated using the firstburst offset as determined by the first FTM request. Conversely, STA2operates according to a second burst period (Burst 2) calculated using asecond burst offset as determined by the second FTM request, resultingin a burst period mismatch.

Systems and methods according to the principles of the presentdisclosure prevent burst period mismatches caused by mismatches betweenFTM requests and FTM responses. For example, the FTM systems and methodsmay implement an absolute burst start time instead of using burstoffsets. Although FTM systems and methods are described, the principlesof the present disclosure may be applied to other time of flight (ToF)or range/distance measurement procedures.

FIG. 4 shows an example WLAN 100 including one or more wirelesscommunication devices configured to implement systems and methodsaccording to an embodiment of the present disclosure. The WLAN 100includes an access point (AP) 104 having a host processor 108 incommunication with a network interface 112. The network interface 112includes a medium access control (MAC) device 116 and a physical layer(PHY) device 120. The PHY device 120 includes a plurality oftransceivers 124-1, 124-2, . . . , and 124-n, referred to collectivelyas transceivers 124. The transceivers 124 communicate with respectiveantennas 128-1, 128-2, . . . , and 128-n, referred to collectively asantennas 128.

The AP 104 communicates with one or more client stations 132. The clientstation 132 includes a host processor 136 in communication with anetwork interface 140. The network interface 140 includes a MAC device144 and a PHY device 148. The PHY device 148 includes a plurality oftransceivers 152-1, 152-2, . . . , and 152-n, referred to collectivelyas transceivers 152. The transceivers 152 communicate with respectiveantennas 156-1, 156-2, . . . , and 156-n, referred to collectively asantennas 128. Although the WLAN 100 is described with respect tocommunication between an AP and a client station, the principles of thepresent disclosure also correspond to communication between APs,communication between client stations, etc.

The host processor 108, the MAC device 116, and/or the PHY device 120 ofthe AP 104 may be configured to transmit and receive FTM request,response, and acknowledgment frames according to the principles of thepresent disclosure. The transceivers 124 are configured to transmit thedata/management/control frames via the respective antennas 128. Theclient station 132 is also configured to transmit and receive FTMrequest, response, and acknowledgment frames according to the principlesof the present disclosure (e.g., via antennas 156). Each of the AP 104and the client station 132 is configured to perform functions of an FTMprocedure according to the principles of the present disclosure.

FIG. 5 shows example wireless communication devices (e.g., an AP and aclient station, an AP and an AP, a client station and a client station,a station that is not associated with an AP and a station that is notassociated with an AP, etc.) 200, 204 in detail. The devices 200, 204each include a transceiver 208 (e.g., corresponding to one of thetransceivers 128 or 152 as shown in FIG. 4), an FTM module 212, and atiming module 216. The transceiver 208 includes a transmitter 220 and areceiver 224. While only one antenna is shown, the devices 200, 204 mayinclude multiple antennas (e.g., antennas arranged in a MIMOconfiguration) as shown in FIG. 4. The functions and operations of eachof these modules of the devices 200, 204 are described below in detailwith further reference to FIG. 5 and reference to FIGS. 6-11.

The FTM module 212 and the timing module 216 implement an FTM procedureaccording to the principles of the present disclosure. For example only,the FTM module 212 and the timing module 216 may implemented in one ormore various components of the devices 200, 204, including, but notlimited to, a PHY, MAC, or SME sublayer, a baseband processor, etc. TheFTM module 212 performs various functions related to FTM negotiationbetween devices 200, 204, including, but not limited to, generation ofFTM request, response, and acknowledgement frames for transmission andprocessing of received FTM request, response, and acknowledgementframes. The timing module 216 performs functions related to timing andtiming calculations of the FTM procedure, such as providing timingmeasurements and calculations to the FTM module 212.

In an example embodiment, the FTM module 212 performs the FTM procedurebased on an absolute burst start time (or, “burst start time”).Specifically, the devices 200, 204 determine a start of a burst periodbased on the absolute burst start time instead of a burst offset.

For example, in an embodiment, the requesting device (i.e., the devicethat transmits the FTM request) may include the burst start time,instead of the burst offset, in the FTM request. As shown in FIG. 5, theFTM module 212 of the requesting device may receive a burst start timefrom the timing module 216 and generate an FTM request including theburst start time. The burst start time may be selected according to, forexample only, a common time shared between the requesting device and thereceiving device. The receiving device (i.e., the device that receivesthe FTM request) may accept the burst start time provided by therequesting device or provide a different burst start time. For example,the FTM module 212 of the receiving device may accept the burst starttime (i.e., continue with the FTM procedure according to the burst starttime received in the FTM request) or receive a different burst starttime from the timing module 216. The different burst time may beselected according to, for example only, the common time shared betweenthe requesting device and the receiving device. The FTM module 212 ofthe receiving device may then generate an FTM response including thedifferent burst start time received from the timing module 216.

In another embodiment, the requesting device may not include a burststart time in the FTM request. Instead, the receiving device may simplyprovide an FTM response including a burst start time as received fromthe timing module 216 of the receiving device. For example only, theburst start time may be selected according to, for example only, thecommon time shared between the requesting device and the receivingdevice. For example only, the burst time may also be selected accordingto, for example only, the time the event occurred in the receivingdevice.

In any described example embodiment, the burst start time may correspondto a value of a timer implemented by the timing module 216. For example,the value may correspond to an absolute time, which may be measuredusing a reference time (e.g., such as a timing synchronization function(TSF) time). In a WLAN, the TSF is used to maintain synchronization ofall devices in a same basic service set (BSS). Each device maintains itsown TSF time. Devices exchange information (e.g., in beacon frames,etc.) that includes the TSF times of respective devices. Accordingly,devices may set their respective TSF times to a same value as otherdevices to maintain timing synchronization.

In the devices 200, 204, the timing module 216 may implement a timerthat maintains the TSF time. The timing module 216 may provide a burststart time that corresponds to the TSF time of the respective device.For example, the burst start time may correspond to a TSF time equal toa predetermined time subsequent to the device's traffic behavior, powersave behavior, a transmission time of the FTM request, subsequent to areception time of the FTM request, subsequent to a transmission time ofthe FTM response, etc. Accordingly, because each of the devices 200, 204has a synchronized TSF time, each of the devices 200, 204 will use thesame absolute burst start time. The same burst start time may beincluded in each FTM request and FTM response transmitted during the FTMprocedure.

FIGS. 6 and 7 shows an example FTM procedure using an absolute burststart time (e.g., based on a TSF time) according to the principles ofthe present disclosure. STA2 transmits a first FTM request (FTM Request1). The first FTM request may include an absolute burst start time basedon a TSF time of STA2. The absolute burst start time may correspond to atransmission time of the first FTM request. In embodiments, the firstFTM request may not include an absolute burst start time.

STA1 receives the FTM request and transmits a first FTM response (FTMResponse 1) to STA2. The first FTM response includes an absolute burststart time. For example, if the first FTM request included an absoluteburst start time provided by STA2, the first FTM response may includethe absolute burst start time provided by STA2 or may include adifferent absolute burst start time (e.g., based on STA1's scheduledbehavior in the future, traffic behavior, power save behavior). If thefirst FTM request did not include an absolute burst start time, thefirst FTM response will include an absolute burst start time provided bySTA1.

Subsequent FTM requests and responses include whichever absolute burststart time was ultimately provided by STA1 in the first FTM response.Accordingly, even if the MAC of STA1 is not able to transmit the firstFTM response (e.g., due to the transmission medium being busy), STA1 andSTA2 still implement the FTM procedure using the same absolute burststart time. For example, if the first FTM request times out at STA2,causing STA2 to transmit a second FTM request (FTM Request 2), the firstFTM response subsequently received by STA2 still includes the sameabsolute burst start time selected by STA1 in response to the first FTMrequest. Similarly, if STA1 transmits a second FTM response (FTMResponse 2) in response to the second FTM request, the second FTMresponse also includes the absolute burst start time selected by STA1 inresponse to the first FTM request. As shown in FIG. 6, STA1 transmits asecond FTM response. As shown in FIG. 7, STA1 discards the second FTMrequest and does not transmit a second FTM response. As such, both STA1and STA2 continue with the FTM procedure with a burst period (Burst 1)that starts at the same time. In embodiments, a length of the FTMrequest timeout may also be increased (e.g., to greater than 10 ms, suchas 512 ms, 1 s, etc.).

Respective TSF times of the devices 200, 204 may not be the same.Typically, one device updates its TSF time according to the TSF time ofthe other device. For example, if a first device is an AP or group owner(GO) and a second device is a station or client, then the second deviceupdates its TSF time using the TSF time of the first device. In otherwords, the second device adopts the TSF time of the first device tomaintain synchronization. Accordingly, the absolute burst start timecorresponds to the TSF time of the AP or GO device.

Conversely, when each of the first device and the second device is astation or a client, neither TSF time corresponds to the TSF time of anAP or GO device, and therefore neither TSF time is adopted by thestation or client device by default. In embodiments of the presentdisclosure, if both devices are stations or clients, the requestingstation (e.g., STA2) uses the TSF time of the receiving station (e.g.,STA1) as the reference time for the absolute burst start time and FTMprocedure measurements. For example, STA1 transmits an FTM responseincluding a burst start time corresponding to the TSF time of STA1.Accordingly, STA2 performs calculations related to FTM based on the TSFtime (as represented by the burst start time) of STA1. During asubsequent burst period, STA1 and STA2 preferably do not change theirrespective TSF times to avoid affecting FTM measurements. If STA1 doeschange its TSF time, STA1 may provide an updated TSF time to STA2 viaanother FTM response.

Similarly, when each of the first device and the second device is an APor GO, the requesting AP (e.g., AP2) uses the TSF time of the receivingAP (e.g., AP1) as the reference time for the absolute burst start timeand FTM procedure measurements. For example, AP1 transmits an FTMresponse including a burst start time corresponding to the TSF time ofAP1. Accordingly, AP2 performs calculations related to FTM based on theTSF time (as represented by the burst start time) of AP1. During asubsequent burst period, AP1 and AP2 preferably do not change theirrespective TSF times to avoid affecting FTM measurements. If AP1 doeschange its TSF time, AP1 may provide an updated TSF time to AP2 viaanother FTM response.

FIG. 8 shows an example method 300 for performing an FTM procedureaccording to the principles of the present disclosure. The method 300begins at 304. At 308, the method 300 transmits a first FTM request. Forexample, an FTM module of a requesting station (STA2) generates thefirst FTM request to be transmitted from STA2. The first FTM request mayinclude an absolute burst start time selected by STA2. The absoluteburst start time may correspond to a TSF time of STA2. At 312, areceiving station (STA1) receives the first FTM request. At 316, STA1selects either the burst start time in the first FTM request or a burststart time based on as TSF time of STA1.

At 320, STA1 attempts to transmit a first FTM response including theselected burst start time. At 324, the method 300 determines whether theattempt to transmit the first FTM response was successful. For example,STA1 may not be able to transmit the first FTM response if thetransmission medium is busy. If true, the method 300 continues to 328.If false, the method 300 continues to 332. At 332, the method 300determines whether the first FTM request timed out. For example, STA2may determine that the first FTM request timed out if the first FTMresponse is not received within a predetermined period. If true, themethod 300 continues to 336. If false, the method 300 continues to 320.At 336, STA2 may transmit a second FTM request and the method 300continues to 320. At 320 the method 300 may continue to attempt totransmit the first FTM response from STA1. In embodiments, STA1 maydiscard the second FTM request or attempt to transmit a second FTMresponse in addition to the first FTM response.

At 328, STA2 receives the first FTM response including burst start timeselected by STA1. At 340, STA1 and STA2 continue with the FTM procedureand corresponding measurements using the burst start time selected bySTA1. The method 300 ends at 344.

The wireless communications described in the present disclosure can beconducted in full or partial compliance with IEEE standard 802.11-2012,IEEE standard 802.16-2009, IEEE standard 802.20-2008, and/or BluetoothCore Specification v4.0. In various implementations, Bluetooth CoreSpecification v4.0 may be modified by one or more of Bluetooth CoreSpecification Addendums 2, 3, or 4. In various implementations, IEEE802.11-2012 may be supplemented by draft IEEE standard 802.11ac, draftIEEE standard 802.11ad, and/or draft IEEE standard 802.11ah.

The foregoing description is merely illustrative in nature and is in noway intended to limit the disclosure, its application, or uses. Thebroad teachings of the disclosure can be implemented in a variety offorms. Therefore, while this disclosure includes particular examples,the true scope of the disclosure should not be so limited since othermodifications will become apparent upon a study of the drawings, thespecification, and the following claims. As used herein, the phrase atleast one of A, B, and C should be construed to mean a logical (A OR BOR C), using a non-exclusive logical OR, and should not be construed tomean “at least one of A, at least one of B, and at least one of C.” Itshould be understood that one or more steps within a method may beexecuted in different order (or concurrently) without altering theprinciples of the present disclosure.

In this application, including the definitions below, the term ‘module’or the term ‘controller’ may be replaced with the term ‘circuit.’ Theterm ‘module’ may refer to, be part of, or include: an ApplicationSpecific Integrated Circuit (ASIC); a digital, analog, or mixedanalog/digital discrete circuit; a digital, analog, or mixedanalog/digital integrated circuit; a combinational logic circuit; afield programmable gate array (FPGA); a processor circuit (shared,dedicated, or group) that executes code; a memory circuit (shared,dedicated, or group) that stores code executed by the processor circuit;other suitable hardware components that provide the describedfunctionality; or a combination of some or all of the above, such as ina system-on-chip.

The module may include one or more interface circuits. In some examples,the interface circuits may include wired or wireless interfaces that areconnected to a local area network (LAN), the Internet, a wide areanetwork (WAN), or combinations thereof. The functionality of any givenmodule of the present disclosure may be distributed among multiplemodules that are connected via interface circuits. For example, multiplemodules may allow load balancing. In a further example, a server (alsoknown as remote, or cloud) module may accomplish some functionality onbehalf of a client module.

The term code, as used above, may include software, firmware, and/ormicrocode, and may refer to programs, routines, functions, classes, datastructures, and/or objects. The term shared processor circuitencompasses a single processor circuit that executes some or all codefrom multiple modules. The term group processor circuit encompasses aprocessor circuit that, in combination with additional processorcircuits, executes some or all code from one or more modules. Referencesto multiple processor circuits encompass multiple processor circuits ondiscrete dies, multiple processor circuits on a single die, multiplecores of a single processor circuit, multiple threads of a singleprocessor circuit, or a combination of the above. The term shared memorycircuit encompasses a single memory circuit that stores some or all codefrom multiple modules. The term group memory circuit encompasses amemory circuit that, in combination with additional memories, storessome or all code from one or more modules.

The term memory circuit is a subset of the term computer-readablemedium. The term computer-readable medium, as used herein, does notencompass transitory electrical or electromagnetic signals propagatingthrough a medium (such as on a carrier wave); the term computer-readablemedium may therefore be considered tangible and non-transitory.Non-limiting examples of a non-transitory, tangible computer-readablemedium are nonvolatile memory circuits (such as a flash memory circuit,an erasable programmable read-only memory circuit, or a mask read-onlymemory circuit), volatile memory circuits (such as a static randomaccess memory circuit or a dynamic random access memory circuit),magnetic storage media (such as an analog or digital magnetic tape or ahard disk drive), and optical storage media (such as a CD, a DVD, or aBlu-ray Disc).

The apparatuses and methods described in this application may bepartially or fully implemented by a special purpose computer created byconfiguring a general purpose computer to execute one or more particularfunctions embodied in computer programs. The functional blocks andflowchart elements described above serve as software specifications,which can be translated into the computer programs by the routine workof a skilled technician or programmer.

The computer programs include processor-executable instructions that arestored on at least one non-transitory, tangible computer-readablemedium. The computer programs may also include or rely on stored data.The computer programs may encompass a basic input/output system (BIOS)that interacts with hardware of the special purpose computer, devicedrivers that interact with particular devices of the special purposecomputer, one or more operating systems, user applications, backgroundservices, background applications, etc.

The computer programs may include: (i) descriptive text to be parsed,such as HTML (hypertext markup language) or XML (extensible markuplanguage), (ii) assembly code, (iii) object code generated from sourcecode by a compiler, (iv) source code for execution by an interpreter,(v) source code for compilation and execution by a just-in-timecompiler, etc. As examples only, source code may be written using syntaxfrom languages including C, C++, C #, Objective C, Haskell, Go, SQL, R,Lisp, Java®, Fortran, Perl, Pascal, Curl, OCaml, Javascript®, HTML5,Ada, ASP (active server pages), PHP, Scala, Eiffel, Smalltalk, Erlang,Ruby, Flash®, Visual Basic®, Lua, and Python®.

None of the elements recited in the claims are intended to be ameans-plus-function element within the meaning of 35 U.S.C. § 112(f)unless an element is expressly recited using the phrase “means for,” orin the case of a method claim using the phrases “operation for” or “stepfor.”

What is claimed is:
 1. A first wireless communication device in awireless network including the first wireless communication device and asecond wireless communication device, the first wireless communicationdevice comprising: a timing module to provide a timing synchronizationfunction (TSF) time, wherein values of the TSF time correspond to acommon time shared between the first wireless communication device andthe second wireless communication device; and a fine timing measurementmodule to transmit a first request to perform a fine timing measurementto determine a distance between the first wireless communication deviceand the second wireless communication device, wherein the first requestincludes a first value of the TSF time at which to start a burst periodof the fine timing measurement, and wherein the first value isdetermined in accordance with the TSF time provided by the timingmodule, receive a first response as transmitted from the second wirelesscommunication device in response to the first request, wherein the firstresponse includes the first value of the TSF time, and transmit a secondrequest to start the burst period of the fine timing measurement at thefirst value of the TSF time as included in the first request.
 2. Thefirst wireless communication device of claim 1, wherein the fine timingmeasurement module is further configured to receive a second responsetransmitted from the second wireless communication device in response toa determination that transmission of the first response wasunsuccessful, wherein the second response includes a same first value ofthe TSF time.
 3. The first wireless communication device of claim 1,wherein the fine timing measurement module is further configured to (i)transmit a second request to perform the fine timing measurement and(ii) receive a second response including the first value of the TSFtime.
 4. The first wireless communication device of claim 1, furthercomprising a transmitter to transmit the first request from the firstwireless communication device and a receiver to receive the firstresponse transmitted from the second wireless communication device. 5.The first wireless communication device of claim 1, wherein the timingmodule includes a timer that maintains the TSF time.
 6. The firstwireless communication device of claim 1, wherein the first valuecorresponds to a predetermined time subsequent to a time that the firstrequest is transmitted.
 7. The first wireless communication device ofclaim 1, wherein the fine timing measurement module is furtherconfigured to calculate the first value based on scheduled communicationbehavior of the first wireless communication device.
 8. A systemcomprising the first wireless communication device of claim 1 andfurther comprising the second wireless communication device.
 9. Thesystem of claim 8, wherein the second wireless communication device isconfigured to (i) receive the first request from the first wirelesscommunication device and (ii) perform the fine timing measurement in theburst period starting at the TSF time indicated by the first valueincluded in the first request.
 10. A method of operating a firstwireless communication device in a wireless network including the firstwireless communication device and a second wireless communicationdevice, the method comprising: providing, at the first wirelesscommunication device, a timing synchronization function (TSF) time,wherein values of the TSF time correspond to a common time sharedbetween the first wireless communication device and the second wirelesscommunication device; transmitting, from the first wirelesscommunication device, a first request to perform a fine timingmeasurement to determine a distance between the first wirelesscommunication device and the second wireless communication device,wherein the first request includes a first value of the TSF time atwhich to start a burst period of the fine timing measurement, andwherein the first value is determined in accordance with the providedTSF time, receiving, at the first wireless communication device, a firstresponse as transmitted from the second wireless communication device inresponse to the first request, wherein the first response includes thefirst value of the TSF time, and transmitting a second request to startthe burst period of the fine timing measurement at the first value ofthe TSF time as included in the first request.
 11. The method of claim10, further comprising receiving, at the first wireless communicationdevice, a second response transmitted from the second wirelesscommunication device in response to a determination that transmission ofthe first response was unsuccessful, wherein the second responseincludes a same first value of the TSF time.
 12. The method of claim 10,further comprising (i) transmitting, from the first wirelesscommunication device, a second request to perform the fine timingmeasurement and (ii) receiving, at the first wireless communicationdevice, a second response including the first value of the TSF time. 13.The method of claim 10, further comprising maintaining the TSF timeusing a timer.
 14. The method of claim 10, wherein the first valuecorresponds to a predetermined time subsequent to a time that the firstrequest is transmitted.
 15. The method of claim 10, further comprisingcalculating the first value based on scheduled communication behavior ofthe first wireless communication device.